Results of charge-density studies of Cu20 have been unimpressive for years, and the agreement between different research groups has been marginal. In this work, the influence of data-reduction procedures, temperature-factor models and deformation models on the results is studied with the aim to identify robust features. Eight deformation-density refinements have been carried out with respect to data measured with Ag Ks radiation to (sin 0/A)max = 1.73/~-~. It is shown that standard Fourier methods lead to unsatisfactory results, since ~ of the structure factors are too small to be measured. Model maps reproducibly show a density deficit along the linear O-Cu-O bond axis. This feature corresponds to the only robust variable, the population coefficient of the Y4o-hexadecapolar function of Cu. Inclusion of anharmonic displacement parameters results in bonds with cylindrical symmetry. The maps and the interpretation in terms of an orbital-product formalism are in good agreement with modern theoretical work: Cu ~+ ions with electron configuration dS(ds) 2 in the field of 0 2-ions form a~b(3dzQ + fl~b(4s) hybrid orbitals, with 0.7 (2). The electric field gradient at the site of Cu cannot be computed from the X-ray data.The electric field gradient tensor at the site of Cu has axial symmetry and is thus completely described by one component, VEz~ =-(a2V/az 2) (z along 3, V = electrostatic potential). The quadrupole coupling
Deoxy-hydroxylamino-sugar Derivatives and Corresponding Diglycosylnitroxides Radicals
SummaryA number of sugar aldonitrones, including C, N-diglycosylnitrones, and ketonitrones have been treated with Grignard reagents or cyanide anion leading to the corresponding deoxy-hydroxylamino-sugars. On oxidation (air, H,IO, or Pb02), these compounds gave the corresponding nitroxide radicals whose ESR. spectra are reported. Analogues of disaccharides, in which the interglycosidic 0-bridge is replaced by a hydroxyirnino group, have been obtained by reacting a partially blocked sugar bearing a free hemiacetal group either with a deoxy-hydroxylarninosugar or with hydroxylamine, followed by reaction with an aldehydosugar and a reducing agent (NaBH4). These reactions represents the key synthetic steps for the oligosaccharide-type synthesis of deoxy-hydroxyimino-oligosaccharides. Their oxidation yielded the corresponding nitroxide radicals whose ESR. spectra gave information on the conformation about the 'interglycosidic' bridge. This type of compounds should constitute useful spin markers for biological studies.A l'exception de deux exemples isoles que nous avons decrits dans une prtcedente communication [4], les analogues de di-ou oligosaccharides dans lesquels le pont oxygene interglycosidique habitue1 est remplacee par un pont hydroxyimino etaient inconnus. Des composes de ce type, de structure proche de celle d'oligosaccharides immunocompetents, devraient &tre appelCs a jouer un r81e important dans la comprehension des mecanismes de reconnaissance cellulaire ou tissulaire. En effet leur facile oxydation en radicaux diglycosylnitroxyde leur permet I )
2,~~
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.